The present invention relates to a method for growing nitride semiconductor crystals, a nitride semiconductor device and a method for fabricating the same.
Nitride semiconductors such as GaN, InN and AlN are materials suitably used for blue-light-emitting semiconductor laser devices and numerous types of semiconductor devices, e.g., transistors operating at a high speed at an elevated temperature.
Various methods have been suggested to form a single crystal layer of a nitride semiconductor suitable for these semiconductor devices.
For example, according to a conventional technique, a nitride semiconductor layer (e.g., an AlN layer) is directly deposited on a single crystal substrate of sapphire (Al.sub.2 O.sub.3) or Si by a metalorganic vapor phase epitaxy (abbreviated to "MOVPE" and also called a "metalorganic chemical vapor deposition (MOCVD)") process. The nitride semiconductor layer formed by this method, however, has poor surface morphology and is likely to crack, resulting in a lower yield. Thus, this method has not been put into practice. Cracking is probably caused due to a thermal stress resulting from a difference in thermal expansion coefficient between a single crystal substrate and a nitride semiconductor layer during the process of lowering the deposition temperature of the nitride semiconductor layer (about 1000.degree. C. for AlN) to room temperature.
Another technique of forming a single crystalline nitride semiconductor layer was developed later as disclosed in Japanese Laid-Open Publications Nos. 4-297023 and 7-312350. According to this technique, an amorphous or polycrystalline nitride semiconductor layer (i.e., a GaN or GalaAlaN (where 0&lt;a.ltoreq.1) layer) is once formed on a single crystal substrate of sapphire or silicon at a relatively low temperature by an MOVPE process. Thereafter, the nitride semiconductor layer is heated to form a partially single crystalline buffer layer and then nitride semiconductor layers for a semiconductor device are epitaxially grown on the buffer layer.
A light-emitting device disclosed in Japanese Laid-Open Publication No. 6-177423 is known as an exemplary semiconductor device using a nitride semiconductor layer formed on a buffer layer. As shown in FIG. 14, this light*emitting device 900 includes: a buffer layer 95 of polycrystalline or amorphous GaN or Ga.sub.1-a Al.sub.a N (where 0&lt;a.ltoreq.1); an n-type Ga.sub.1-b Al.sub.b N (where 0.ltoreq.b&lt;1) cladding layer 96; an n-type In.sub.x Ga.sub.1-x N (where 0&lt;x&lt;0.5) active layer 97; and a p-type Ga.sub.1-c Al.sub.c N (where 0.ltoreq.c&lt;1) cladding layer 98, which are stacked in this order on a sapphire substrate 92.
The crystal growing technique for the buffer layer 95 is also disclosed in Japanese Laid-Open Publications Nos. 4-297023 and 7-312350 identified above. Specifically, according to the method disclosed in these references, GaN or Ga.sub.1-a Al.sub.a N (where 0&lt;a.ltoreq.1) crystals are grown at a temperature ranging from 200.degree. C. to 900.degree. C., both inclusive, by an MOVPE process to form the buffer layer 95. In accordance with this method, part of the buffer layer 95 is turned into single crystals during a process of raising the temperature after the buffer layer 95 of polycrystalline Ga.sub.1-a Al.sub.a N (where 0&lt;a.ltoreq.1) has been deposited on the sapphire substrate 92 at a low temperature and before a nitride semiconductor crystal layer, e.g., the n15 type Ga.sub.1-b Al.sub.b N (where 0.ltoreq.b&lt;1) cladding layer 96, is deposited at a temperature of about 1000.degree. C.
The present inventors minutely analyzed the cross-section of nitride semiconductor crystals, which had been grown on a sapphire substrate at a low temperature by the conventional technique, using a transmission electron microscope. As a result, we found that the nitride semiconductor crystal layer, which had been formed by the prior art crystal growing technique, had a lot of dislocations and that the lifetime of a semiconductor device including such a nitride semiconductor layer was short.
In the conventional method for fabricating a semiconductor device, it seems to be only a small region of the buffer layer 95 within a plane of the sapphire substrate 92 that is turned into single crystals during the temperature raising process before the nitride semiconductor crystal layers are grown. Thus, it is considered that, in the remaining region of the buffer layer 95 that is not turned into single crystals, the polycrystals have poorly aligned orientations to generate a large number of dislocations (or other defects) in the interface between the sapphire substrate 92 and the buffer layer 95. And such dislocations would grow to reach the nitride semiconductor crystal layers (i.e., the cladding layer 96, active layer 97 and cladding layer 98 in this case). We found that the density of dislocations in the nitride semiconductor crystal layers was as high as 10.sup.9 cm.sup.-2, thus adversely shortening the life of the semiconductor device.
Still another technique of forming an AlN buffer layer by nitrifying (in this specification, to "nitrify" means "to combine with nitrogen or its compounds") the surface of a sapphire single crystal substrate was suggested in Japanese Laid-Open Publication No. 63-178516, for example. In accordance with this technique, however, the buffer layer is also likely to crack or a lot of dislocations are also created in the buffer layer as in the prior art method just described. Thus, this technique has not been put into practice, either.